Measurement of steel reinforcement corrosion in concrete is essential to analyze the strength and durability of a structure. Not only does the steel corrosion reduce the life span of the structure but also increases the cost for inspection and maintenance significantly.
Currently, no instrument or technique is available which can measure the extent of corrosion of steel. However, measurement of concrete properties, such as resistivity and half-cell potential of concrete can assess the probability of corrosion of reinforcing steel.
Basis for Corrosion Measurement of Rebars
Commonly, ions penetrate into concrete through pores which render concrete to act as an electrolyte. In this condition, the corrosion of steel bars embedded in concrete would occur.
As electrons move in concrete, an electrical potential field inside concrete over steel bars is generated. The electrical potential field is the foundation for studying the corrosion of steel bars.
The majority of corrosion measurement techniques are based on examining the electrochemical condition of the rebar-concrete interface. This examination is generally conducted from the surface of concrete.
Different techniques have been developed and used for measurement of steel corrosion, for instance- Resistivity meter, Half-cell potential, and iCOR®. These are discussed in the following paragraphs.
The corrosion of steel in concrete is an electrochemical process that generates a flow of current. The resistivity of the concrete influences the flow of this current. The lower the electric resistance, the more easily corrosion current flows through the concrete and the greater is the probability of corrosion.
Thus, the resistivity of concrete is a good indication of the probability of corrosion. Resistivity meter can be used to assess the probability of corrosion of steel reinforcement embedded in concrete. It is a very simple technique and can be adopted easily in the field.
Resistivity meter is very handy and portable equipment weighing about 2.2 Kg. It has two or more probes, which are placed on a concrete surface with conductive gel between probes and surface. The concrete resistivity is displayed on an LCD.
Nowadays, resistivity meters are available with non-volatile memory and colored graphic display from which data can be transferred on PC. To measure the resistivity, metallic probes are placed over the concrete surface. A known current is passed on the outer probes and resulting potential drop between inner probes is measured.
The resistance is computed by dividing the potential drop by the current. A conductive gel is used between probe and concrete surface to make effective contact. The probable rate of corrosion with respect to the value of resistivity of concrete is normally considered as given in the table below.
Table 1 Resistivity Level Versus Possible Corrosion Rate of Steel Reinforcements in Concrete
|Resistively level (Kilo-ohm / cm)||Possible corrosion rate|
|< 5||Very high|
|5 to 10||High|
|10 to 20||Moderate to low|
Hall-cell Potential Test
The difference in potential between concrete surface and steel is a good indicator of current flow. The electrochemical process produces an electric current, which is measurable as an electric field on the surface of the concrete.
This potential field can be measured with an electrode known as half-cell potential test which is standardized by ASTM international standard. By making measurement over the whole surface, a distinction can be made between likely corroding and non-corroding locations. The probability of corrosion with respect to the values of potential difference is normally considered as given in table 2 below:
Table 2 Value of Hall-cell Potential Test Versus the possible Rate of Corrosion of Steel bars in Concrete
|Potential Value||Possible Corrosion Rate|
|<= 0.20 V||90% probability of no corrosion|
|0.20 to -0.35 V||Corrosion activity uncertain|
|> 0.35 V||more than 90% probability of corrosion|
Hall-cell potential test equipment is handy and weighs about 5.5 Kg with large display and simple operation. Measured values can be represented on the display. Measurements can be stored in the memory. Its data can be transferred to PC.
In order to be able to conduct this test, the steel in concrete structure should be accessible at few locations to provide electrical connection, as it can be observed from Fig. 2. For new structures, such locations should be decided at the design stage itself. The connections project out of the concrete. For existing bridges, re-bars/prestressing wires are to be exposed to make electrical connections.
Commonly, bores are drilled at desired locations to expose steel bars embedded in concrete, and then an electric cable is connected to the steel bars. After connecting them from outside, the same can be plugged back using epoxy mortar. The positive terminal of the voltmeter is connected to exposed rebars and the negative terminal (common) to reference half-cell.
The surface of concrete is divided into number of grids. The reference electrode is moved along the nodal point and corresponding potentials are recorded. These are referred to as corrosion potential.
iCOR® Test Equipment
It is a cost-effective and non-destructive test tool which is capable to perform three tests on concrete namely: electrical resistivity test, hall-cell potential test, and steel bar corrosion rate.
This device uses Connectionless Electrical Pulse Response Analysis (CEPRA) technology to operate, and able to measure the electrical response of the reinforcement inside the concrete without a physical connection to the rebar. So, there is no need to drill holes in concrete to expose steel bars and connect them to corrosion measuring devices as it is the case in tests discussed above.
- Set up the specification of the structure in the app which is installed on the iCOR® previously.
- After that, conduct a measurement on a specific location of the grid by moving the device on the concrete surface to the corresponding grid points on your tablet.
- Then iCOR® instrument communicates these measurements to your app which processes the information and outputs concrete corrosion results in real-time.
- This data can be viewed as a corrosion rate map, in a PDF report, and CSV data file.